Quite Great
The Gameplay:I played it like I played Bullet Heaven and it wasn't too awesome(even though the mp3 using system is awesome because I rarely see any game has that feature),but the only flaw is that when you get the Level 4 upgrade of the bullets,shooting sideways seems really bad because it only hits when you shoot at the middle of the boss(in the boss).
the graphics wasn't too extraordinary,not bad either.It's a good thing the background changes between levels.However,I found it to be laggy on very slow computers,so I think an option to lower the game quality(like most flash games have)would be lag reducing.
the controls were never a problem to me.never.except the absence of the mouse control makes the game a little bad because I like using the mouse rather than the keyboard(Of course if you do make mouse controls try to limit the speed!).
Other suggestions:Make an option to change whether the background moves or not since i'm sure this is a cause of lag.
It would be better if the characters had more than one ability(example:the bombs can reflect the bosses/enemies bullets rather than just the bomb taking all the bullets.Add this to every single character so no mode will be less played.
This also might be a good reason to add more characters.also try to make the bullet upgrades different for each character.
Make unlockable cheats(Heh....)and mods make really unfair cheats like infinite health causes the player not able to submit his/her score into the high score boards.On the other hand,mods that make the game harder like the bullets being invisible after launched by the enemy after a while.
Maybe if the keyboard controls were customizeable it would be better.
Of course make enemies before you fight the boss(or something like that).
Sorry for my maybe bad English.